Stadium put up in a number of years (5)
I believe the answer is:
arena
'stadium' is the definition.
(synonyms)
'in a number of years' is the wordplay.
'in' is an insertion indicator.
'a' becomes 'area' (maths abbreviation).
'number of years' becomes 'n' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'area' enclosing 'n' is 'ARENA'.
'put up' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for arena that I've seen before include "Place of activity, especially conflict" , "Central area in sports stadium" , "park" , "place involving conflict?" , "where there's competition" .)